Sintered steel alloy for wear resistance at high temperatures and fabrication method of valve-seat using the same

ABSTRACT

Disclosed is a sintered steel alloy for wear resistance at high temperatures, which is applied to a valve seat of an internal combustion engine including an automobile. The sintered steel alloy includes: 10.0 to 14.0 parts by weight of cobalt powder; 5.0 to 9.0 parts by weight of molybdenum powder; 1.5 to 4.1 parts by weight of chromium powder; 0.7 to 1.3 parts by weight of carbon powder; 1.0 to 1.8 parts by weight of manganese powder; 0.4 to 1.2 parts by weight of silicon powder; 0.2 to 0.8 parts by weight of sulfur powder; and 0.1 to 0.7 parts by weight of vanadium powder, based on 100 parts by weight of iron powder, and thus a service life of the valve seat is extended.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2018-0104490, filed on Sep. 3, 2018, the disclosureof which is incorporated herein by reference in its entirety.

BACKGROUND 1. Field of the Invention

The present invention relates to a sintered steel alloy for wearresistance at high temperatures, which is applied to a valve seat for aninternal combustion engine, and more particularly, to a sintered steelalloy for wear resistance at high temperatures, in which a compositionof the sintered steel alloy is changed to maximize its heat resistanceas well as its wear resistance, and a method of manufacturing a valveseat using the same.

2. Discussion of Related Art

In general, a valve seat for an internal combustion engine is aring-shaped sintered body, which maintains the airtightness ofintake/exhaust valves in an opening/closing process of the intake valveand the exhaust valve. As shown in FIG. 1, a valve seat 18 performs asealing function by contacting head portions of the intake valve and theexhaust valve while the valve seat 18 is inserted into a cylinder head24.

The valve seat 18 contacts the intake/exhaust valves in a closing strokeof the exhaust valve, and performs a function of preventing leakage ofexhaust gas in an opening stroke of the exhaust valve. Thus, insintering the valve seat 18, there is a demand that physicalrequirements such as wear resistance or heat resistance be sufficient towithstand continuous friction with the intake/exhaust valves andcontinuous chemical reactions with the exhaust gas.

Accordingly, there is much ongoing research on improving the wearresistance and heat resistance of the valve seat. As an example, it iswidely known that there is a method of dispersing cobalt-based orchromium-based carbides into an iron matrix or a method of dispersinghard particles in a form of ferro-chromium (Fe—Cr)-based orferro-molybdenum (Fe—Mo)-based intermetallic compounds.

As other examples that improve the wear resistance and heat resistanceof the valve seat, it is known that there are an infiltration methodusing a power metallurgy method of adding various kinds of alloys thathave excellent wear resistance and heat resistance into the iron matrix,an addition method of adding hard particles, a manufacturing method bymeans of controlling of a matrix alloy, a sinter forging method, and thelike.

However, the internal combustion engine employs a method of combusting aliquid fuel and a gas fuel as well, and a wider variety of forms ofcombustion products are formed by means of the liquid fuel and the gasfuel, so that there is a need to further reinforce physical conditionssuch as wear resistance and heat resistance to be able to withstand thecombustion products from the liquid fuel and the gas fuel.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1 KR10-2004-0025003 A

Patent Document 2 KR10-2012-0125817 A

Patent Document 3 KR10-0461304 B1

SUMMARY OF THE INVENTION

The present invention is directed to providing a sintered steel alloyfor wear resistance at high temperatures, in which a composition of thesintered steel alloy is changed to maximize its heat resistance as wellas its wear resistance and extend a service life of a valve seat, and amethod of manufacturing a valve seat using the same.

According to one aspect of the present invention, there is provided asintered alloy including: 10.0 to 14.0 parts by weight of cobalt powder;5.0 to 9.0 parts by weight of molybdenum powder ; 1.5 to 4.1 parts byweight of chromium powder; 0.7 to 1.3 parts by weight of carbon powder;1.0 to 1.8 parts by weight of manganese powder; 0.4 to 1.2 parts byweight of silicon powder; 0.2 to 0.8 parts by weight of sulfur powder;and 0.1 to 0.7 parts by weight of vanadium powder, based on 100 parts byweight of iron powder.

According to another aspect of the present invention, there is provideda manufacturing method including: a mixing operation of evenly mixingthe sintered alloy; a pressurizing operation of pressurizing a resultingmixture formed in the mixing operation at a set pressure; a sinteringoperation of sintering a resulting molded body formed in thepressurizing operation along with an infiltrate to infiltrate copperinto the molded body; a low temperature treatment operation of treatinga resulting sintered body formed in the sintering operation at a lowtemperature to change residual austenite into martensite; and a heattreatment operation of tempering a resulting low temperature treatedbody formed in the low temperature treatment operation to remove aresidual stress therefrom.

As described above, the present invention has at least the followingeffects.

Firstly, cobalt, molybdenum, or chromium and a component for increasingstrength are added into the composition of the valve seat to formcomplex carbides, so that the precipitated particles and the amount ofsolid solubility of an iron matrix for the valve seat are increased anda service life of the valve seat is extended.

Secondly, silicon or vanadium is added into the composition of the valveseat to disperse micro-spherical particles into the iron matrix, so thata loss of carbide particles is decreased in an abrasion process of thevalve seat, an amount of abrasion is reduced, and a service life of thevalve seat is extended.

Thirdly, manganese, sulfur and the like are added into the compositionof the valve seat to improve self-lubrication, so that the machinabilityof the valve seat is improved, abrasion is also minimized in a frictionprocess with the intake/exhaust valves, and a service life of the valveseat is further extended.

Lastly, a sintering process is performed while copper powder isinfiltrated into the composition of the valve seat, so that wearresistance and heat resistance are improved in comparison with anexisting valve seat, and the composition of the valve seat may beapplied to any internal combustion engine using a gas fuel as well as aliquid fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent to those of ordinary skill in theart by describing in detail exemplary embodiments thereof with referenceto the accompanying drawings, in which:

FIG. 1 is a diagram illustrating an installation state of a valve seataccording to the related art;

FIG. 2 is an optical microscope picture (500× magnification) of Example1 according to the present invention;

FIG. 3 is an optical microscope picture (500× magnification) of Example2 according to the present invention; and

FIG. 4 is an optical microscope picture (500× magnification) of Example3 according to the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a composition according to the present invention will bedescribed.

As shown in FIGS. 2 to 4, a sintered steel alloy according to thepresent invention represents a sintered alloy applied to an internalcombustion engine, and particularly applied to a valve seat thatmaintains airtightness of intake/exhaust valves in an opening/closingprocess of the intake valve and the exhaust valve and also minimizesdamage in a contact process with combustion products.

Although only application of the sintered steel alloy for wearresistance at high temperatures to a valve seat is described here, itcan also naturally be applied to a cylinder liner, a valve guide or thelike within the same technical scope.

The composition of the sintered steel alloy according to the presentinvention includes a sintered alloy, in which iron powder is a maincomponent, as well as an infiltrate, which is infiltrated into thesintered alloy, and the sintered alloy includes: 10.0 to 14.0 parts byweight of cobalt powder; 5.0 to 9.0 parts by weight of molybdenumpowder; 1.5 to 4.1 parts by weight of chromium powder; 0.7 to 1.3 partsby weight of carbon powder; 1.0 to 1.8 parts by weight of manganesepowder; 0.4 to 1.2 parts by weight of silicon powder; 0.2 to 0.8 partsby weight of sulfur powder; and 0.1 to 0.7 parts by weight of vanadiumpowder, based on 100 parts by weight of the iron powder. The infiltrateis copper powder which amounts to 10.0 to 20.0 parts by weight based on100 parts by weight of the iron powder.

At this time, the infiltrate is infiltrated into the sintered alloy sothat, in the case of the composition of the sintered steel alloy,complex carbides such as a cobalt-based hard particle phase, amolybdenum-based hard particle phase or a chromium-based hard particlephase are evenly dispersed in a martensite matrix, particularly anintermetallic compound between the manganese and sulfur or the manganeseand carbon serves as a lubricant, and particles are refined by means ofthe silicon or the vanadium.

In other words, a reason for infiltrating the infiltrate into thesintered alloy to manufacture the valve seat is to further increase heatresistance at high temperatures, wear resistance at high temperaturesand corrosion resistance at contact portions with the intake/exhaustvalves.

The valve seat manufactured of the composition of the sintered steelalloy (hereinafter, collectively referred to as the composition) is amaterial of high strength, in which its final product has a hardness(HRA) of at least 71 to 81 and maintains a density (g/cm³) of at least7.4 to 8.1.

In the meantime, the cobalt (Co) reacts with iron, molybdenum or carbonto precipitate complex carbides, and thus it is evenly dispersed in thematrix and contributes to wear resistance, while a part of the cobalt issolid-solved in the matrix, so that heat resistance is increased. If acontent of the cobalt is less than 10.0 parts by weight, theprecipitated particles and the amount of solid solubility of the matrixare decreased, and thus wear resistance and heat resistance deteriorate.If a content of the cobalt is more than 14.0 parts by weight, a matrixmetal becomes vulnerable due to an excess of precipitated particles, andthus machinability deteriorates.

Also, the molybdenum (Mo) is solid-solved in the matrix or forms anintermetallic compound in a complex carbide state, and thus wearresistance and heat resistance are improved. If a content of themolybdenum is less than 5.0 parts by weight, the amount of solidsolubility of the matrix and intermetallic compounds are decreased, andthus wear resistance and heat resistance deteriorate. If a content ofthe molybdenum is more than 9.0 parts by weight, the amount of solidsolubility of the matrix metal is excessive, and thus causes the matrixmetal to become vulnerable.

Further, the chromium (Cr) is a component that reacts with the carbonwithin the matrix to form complex carbides and improve wear resistance,and is also solid-solved in the matrix to improve heat resistance. Acontent thereof may be 1.5 parts by weight to 4.1 parts by weight.

If a content of the chromium is less than 1.5 parts by weight, an amountof complex carbides is decreased, and thus wear resistance and heatresistance deteriorate. If a content of the chromium is more than 4.1parts by weight, the amount of solid solubility of the matrix metal isexcessive, and thus the product becomes vulnerable.

Moreover, the carbon (C) is a component that is solid-solved ordispersed in the matrix to reinforce the matrix, and that also reactswith the cobalt, chromium or molybdenum to form complex carbides. Thecarbon (C) performs a function of increasing the strength and hardnessof the matrix and also increasing its wear resistance or heatresistance.

If a content of the carbon is less than 0.7 parts by weight, ferrite isexcessively formed in the matrix metal along with pearlite, and thus thematrix is softened and strength and wear resistance deteriorate. If acontent of the carbon is more than 1.3 parts by weight, a carbon residueremaining after forming pearlite forms cementite, and thus the matrixsteel becomes vulnerable.

Also, the manganese (Mn) is a component that reacts with sulfur presentin the iron matrix to form MnS and improves self-lubrication. If acontent of the manganese is less than 1.0 part by weight, the MnS isformed, and thus a function of self-lubrication deteriorates. If acontent of the manganese is more than 1.8 parts by weight, there isconcern of segregation in addition to forming of the MnS.

Further, the silicon (Si) is a component that is added for the purposeof adjusting and refining a crystal grain of the iron matrix and alsoimproving wear resistance or heat resistance. A content of the siliconmay be 0.4 to 1.2 parts by weight.

Moreover, the sulfur (S) is a component that is added into the ironmatrix and dispersed in a grain boundary of the matrix in the form ofMnS. The MnS is not decomposed as a compound at high temperatures butmaintains a stabilized state in a grain boundary of a sintered bodyafter going through a sintering process and deteriorates a frictioncoefficient in a process of processing the final product, and thusmachinability is increased. In particular, a content of the sulfur maybe 0.2 to 0.8 parts by weight.

The manganese and the sulfur may be mixed at a ratio of approximately6:4 so that efficiency is increased according to forming of the MnS.

If a content of the MnS (Mn+S) is less than 1.25 parts by weight, itplays an insignificant role in remaining in the matrix of the sinteredbody. If a content of the MnS (Mn+S) is more than 2.6 parts by weight,the strength of the matrix is weakened, thus causing damage to the valveseat.

Also, the vanadium (V) is a component that is added for the purpose ofadjusting and refining a crystal grain of the iron matrix and alsoimproving heat resistance. A content of the vanadium may be 0.1 to 0.7parts by weight. If the vanadium exceeds the required value, the crystalgrain is coarsened, thus causing destruction of the final product of thevalve seat. Hereinafter, a manufacturing method according to the presentinvention will be described.

First of all, the present invention includes: a mixing operation ofmixing the composition to manufacture a mixture; a pressurizingoperation of pressurizing the mixture; a sintering operation ofsintering a resulting body; a low temperature treatment operation ofchanging residual austenite into martensite; and a heat treatmentoperation of removing a residual stress therefrom.

Also, the mixing operation is an operation of evenly mixing the steelalloy powder, a high speed tool steel powder, a superalloy powder, amanganese sulfide powder, a carbon power and the like in accordance withthe required amount of each in a mixer.

Further, the pressurizing operation is an operation of compressing amixture formed in the mixing operation to mold at a density suitable forthe valve seat, and is also an operation of pressurizing the mixture ata surface pressure of 6 to 10 tons/cm² to improve precision.

Moreover, the sintering operation is an operation of sintering a moldedbody molded in the pressurizing operation in a temperature range of1120±20 C. for 30±10 minutes to form a sintered body, and includes anoperation of infiltrating 10.0 to 20.0 parts by weight of the copperpowder into the sintered body.

If a sintering temperature is less than 1100° C. in the sinteringoperation, powder particles are not smoothly dispersed and a matrixstructure is weakened. If the sintering temperature is more than 1140°C., a crystal grain is coarsened and mechanical properties deteriorate.

In the sintering operation, sintering is performed in a state in which10.0 to 20.0 parts by weight of the copper powder are inserted andcopper particles are infiltrated into the pores of the matrix structure,so that the strength of the matrix is reinforced and a lubrication roleis also increased.

Also, the low temperature treatment operation is an operation ofchanging residual austenite into martensite by cooling the sintered bodyformed in the sintering operation in a temperature range of −120±10° C.for 20±5 minutes, so that the aging of the composition is prevented frombeing changed, a mechanical property is improved, and structuralstability is induced.

Further, the heat treatment operation is an operation of tempering a lowtemperature treated body formed in the low temperature treatmentoperation to remove a residual stress therefrom, and is also anoperation of heating in a temperature range of 600±20 ° C. for 120±10minutes to give toughness to the matrix structure.

Moreover, as a post-processing operation of the heat treatmentoperation, an operation of removing foreign materials like burrs fromthe final product and performing a mechanical processing process such asforging or polishing to obtain a completed product may be included, butdescription thereof will be omitted herein.

The completed product of the valve seat, having gone through theoperations above, has a hardness (HRA) of about 71 to 81 and a density(g/cm³) of about 7.4 to 8.1, and it can be seen that it providesappropriate hardness and density to be used with liquid fuels and solidfuels.

Hereinafter, Examples of the present invention will be described.

TABLE 1 Components Example 1 Example 2 Example 3 (parts by weight)(Sample 1) (Sample 2) (Sample 3) Cobalt powder 12 14 10 Molybdenumpowder 7 9 5 Chromium powder 3 4.1 1.5 Carbon powder 1.0 1.3 0.7Manganese powder 1.5 1.8 1.0 Silicon powder 1.0 1.2 0.4 Sulfur powder0.5 0.8 0.2 Vanadium powder 0.5 0.7 0.1 Copper powder 15 15 15 Ironpowder 100 100 100

First of all, a mixture was manufactured by mixing compositions havingthe composition ratios of Examples 1 to 3 of Table 1 in a mixer, and themixture was pressurized at a surface pressure of 10 tons/cm³, and thensintered and infiltrated at 1120° C. for 30 minutes in a heat treatmentfurnace.

Then, a low temperature treated body was manufactured by quenching asintered body that was subjected to sintering and copper infiltration inthe sintering operation in a temperature range of −120° C. for 20minutes, and then the low temperature treated body was heated in atemperature range of 600° C. for 120 minutes and tempered.

Then, a heat treated body that was subjected to the heat treatmentoperation was drawn out, Samples 1 to 3 were manufactured, and then anabrasion loss was measured using an abrasion tester (Rig Tester,nitrogen atmosphere; 0.2 mm Offset; SUH35+Tuff valve, speed: 3,500 rpm,temperature: 350° C., time: 2 hours). From the results, in the case ofExamples 1 to 3 (Samples 1 to 3), it can be seen that an overallabrasion loss of the valve and valve seat amounts to 48 μm on average,which is appropriate for a material of the valve seat.

In other words, as shown in FIGS. 2 to 4, Samples 1 to 3 showed similardensity and hardness values, and particularly, it can be seen that hardparticles and elements for improving processability were evenlydistributed within the martensite matrix structure.

In particular, it can be seen that the wear resistance and heatresistance of the valve seat were increased when the copper alloy wasfilled into the pores of the matrix structure.

As stated above, the present invention is not limited to the exemplaryembodiments described above, and it will be apparent to those skilled inthe art that various modifications can be made without departing fromthe spirit and scope of the invention as defined by the appended claims,and such modifications fall within the scope of the present invention.

What is claimed is:
 1. A sintered steel alloy for wear resistance athigh temperatures, comprising: 10.0 to 14.0 parts by weight of cobaltpowder; 5.0 to 9.0 parts by weight of molybdenum powder; 1.5 to 4.1parts by weight of chromium powder; 0.7 to 1.3 parts by weight of carbonpowder; 1.0 to 1.8 parts by weight of manganese powder; 0.4 to 1.2 partsby weight of silicon powder; 0.2 to 0.8 parts by weight of sulfurpowder; and 0.1 to 0.7 parts by weight of vanadium powder, based on 100parts by weight of iron powder.
 2. The sintered steel alloy of claim 1,wherein 10.0 to 20.0 parts by weight of copper powder based on 100 partsby weight of the iron powder is further added as an infiltrate into acomposition of the sintered steel alloy.
 3. A method of manufacturing avalve seat using a sintered steel alloy for wear resistance at hightemperatures, the method comprising: a mixing operation of evenly mixingthe sintered steel alloy described in claim 1; a pressurizing operationof pressurizing a resulting mixture formed in the mixing operation at aset pressure; a sintering operation of sintering a resulting molded bodyformed in the pressurizing operation along with the infiltrate describedin claim 2 to infiltrate copper into the molded body; a low temperaturetreatment operation of treating a resulting sintered body formed in thesintering operation at low temperatures to change residual austeniteinto martensite; and a heat treatment operation of tempering a resultinglow temperature treated body formed in the low temperature treatmentoperation to remove a residual stress therefrom.
 4. The method of claim3, wherein the pressurizing operation includes pressurizing thecomposition of the valve seat at a surface pressure of 6 to 10 tons/cm³.5. The method of claim 3, wherein a final product after the heattreatment operation has a hardness (HRA) of 71 to
 81. 6. The method ofclaim 3, wherein a final product after the heat treatment operation hasa density (g/cm³) of 7.4 to 8.1.
 7. The method of claim 3, wherein themolded body is sintered and copper-infiltrated in a temperature range of1120±20° C. for 30±10 minutes in the sintering operation.
 8. The methodof claim 3, wherein the low temperature treatment operation includescooling a sintered body formed in the sintering operation in atemperature range of −120±10° C. for 20±5 minutes.
 9. The method ofclaim 3, wherein the heat treatment operation includes heating a lowtemperature heated body formed in the low temperature treatmentoperation in a temperature range of 600±20° C. for 120±10 minutes.